Fast cache is second tier cache in some storage products. It transparently caches data from hard disk drive (HDD) to solid storage disks (SSD). It is known that SSD has much more powerful performance than HDD.
In the previous implementations of fast cache, if users want to expand fast cache with more SSDs, they have to disable the current fast cache to promote all the data therein to the hard disk, destroy the fast cache, and then rebuild the fast cache with more SSDs. This means hot data which have been promoted previously into the fast cache will be lost and it takes a long time for the hot data to be promoted again in the fast cache. During destruction/creation, the user cannot get any benefit from fast cache. Even worse impact might be incurred to the overall performance of the cache array because more internal inputs/outputs (IOs) are generated during this process.